So, if you have any sort of pedal that provide for an expression-pedal input that is simply a variable resistance, you can always stick one of these puppies on your guitar.
They are called force-sensing resistors. The resistance corresponds to how hard you press on the surface of the pad. The ones I bought from my buddy Steve Daniels at Small Bear (
Potentiometers and Trimpots - Force Sensors - Small Bear Electronics ) have a thin adhesive pad on one side and I would imagine most of them do. You can see from the pic that they come in all sorts of shapes and sizes. The ones I bought have a diameter exactly the size of a dime, so they can be positioned pretty much anywhere that is convenient to you. Off resistance is about 20megohms, but pressing on them can drop the resistance very low, very quickly (datasheet shows response time of 1ms).
I have experimented with using a guitar-mounted photocell to control my Line 6 M5 pedal, in place of an expression pedal, and that does some neat things. But the response time of photocells is less precise, and performance depends on the ambient lighting (covering a photocell with your hand in near dark does not really change the photocell resistance very much). So, in many respects, a force-sensing resistor can provide better, and more consistent response.
I don't know how other pedals work, but the M5 allows you to assign up to 5 parameters to the expression pedal. Put the expression in toe down position and tweak the knobs to what you'd like as toe down. Then put the expression in heel down and adjust the knobs to what you want heel-down to be. Now, sweeping the expressi pedal back and forth will sweep from the one extreme setting to the other. Use of the FSR would work in similar fashion. Leave the pad untouched and set your knobs, then press down on the pad and set them for the other direction. Since it would be hard to both pick and press at once, I gather that whatever the press-down produces in the way of effect change is going to be something you want to do
after you pick.
Bottom line: There may be something special about the RevPad, and I imagine they have tailored it to use of the pressure pad on the guitar, but you don't
necessarily have to use that device to accomplish the same things. You can do it yourself, on the cheap. I'm going to have to make a demo video this week.
ADDENDUM: Here's an illustration of some things one can do with an FSR and a small microcontroller. More complicated than an expression-pedal, but you'll get the idea of what can be done with a pressure pad.